Organophosphorus pesticides are the most commonly used insecticides in US. agriculture. Runoff and leaching of these compounds from cropland lead to contamination of surface water and groundwater. It has been found that these compounds are neurodevelopmentalty toxic at low doses. Existing data are inadequate to enable accurate prediction of persistence of this group of insecticides in the aqueous environment. Therefore, such contaminants are of toxicological concern and there exists a potential for exposure of a large proportion of the population. It is likely that reactions with reduced sulfur species (particularly polysulfides) present in anoxic subregions of coastal water bodies and sediment pore waters could have a significant impact on rates of removal of such contaminants. The reaction rates and the reaction produts of reactions under those conditions are not known and some of the products of these reactions may very well be of toxicological concern. Providing the necessary data is the principal objective of this research. Hence, an additional goal of these studies is to provide data related to the ultimate fate of such compounds by examining their ability to bind covalently to natural organic matter. The long term objective in this research project is to collect data that will allow evaluating the persistence and fate of organophosphorus insecticide contaminants. The results will be useful to EPA for establishing scientifically-based limits on insecticides usage in coastal areas. As a first step, the goals of this proposal are to investigate the reaction of organophosphorus insecticides with reduced sulfur species and identify the reaction products of these reactions. Rates of reaction of reduced sulfur species with a group of phosphorothionate esters insecticides will be determined in welldefined systems, the products will be carefully indentified, and additional experiments will be conducted using more complex matrices of natural sulfidic waters. These experiments will allow the exploration of the influence of the presence of natural organic matter on the fate of these insectide.